Alarms are often used as reminders of tasks that need to be performed. For example, the most basic of alarms are those provided by alarm clocks that sound at preset times. Handheld devices, such as personal digital assistants (PDAs), mobile telephones, and other devices, also provide alarm features that enable users to set alarms at preset times. Handheld devices have the advantage that the users can carry such devices with them wherever they travel, so that the users are able to hear or see the alarm when the alarm is activated.
However, basing an alarm only on time reduces flexibility to a user. An alarm may pertain to a task to be performed by a user at a given location. However, when the alarm is activated at the preset time, the user may be far away from that location. If a user is unable to perform the task close to a time at which the alarm is activated, the user may forget to return to the task at a later point in time, which defeats the purpose of such an alarm.